Methods for applying microstructures to the surface of cylindrical drums by pulsed laser ablation have been well known for many years. The main application is in the gravure and flexography printing industries where lasers are used to create ink carrying pits so the drums are able to transfer images directly or indirectly onto flat paper or polymer films. The techniques used are very well developed with a wide range of lasers used to create pits directly in metal drums or in drums coated with ceramic, rubber or polymer layers. U.S. Pat. No. 5,327,167 describes a machine for ablating pits of variable density on the surface of a printing drum.
The lasers used are usually focused to spots on the drum surface in the diameter range 10 to 100 μm to create pits by direct laser ablation or by ablation of a thin mask followed by chemical etching. Characteristics of the lasers used are that they have single or low mode beams which are focused to circular spots that have Gaussian energy distribution and are of the diameter required, that they emit sufficient energy per pulse to ablate a pit with a single pulse and that they operate at high repetition rate so they can create pits at high speed in order to process drums with large surface areas in reasonable times. The diameters of the pits formed range from a few 10s to a few 100s of microns, the limits being set at the small end by the capacity to pick up ink and at the large end by the resolution of the printing requirement. Pits of the smallest size are made by individual laser shots from a single mode laser with modest energy per pulse and larger pits, often of hexagonal shape, are made by making many small pits adjacent to each other. Large circular pits are made with a single laser pulse from a laser emitting a beam with several modes with higher energy per pulse.
U.S. Pat. No. 6,462,307 discloses a process to create a pit in a single laser shot that is not circular in shape. In this case the primary laser beam is divided into multiple angularly separated sub-beams that are individually adjusted in power and are recombined with small variations in position on the drum surface to form a non-circular, non-Gaussian distribution of energy on the drum surface. A description of this multi-beam profile modulation technique, as well as a review of other more standard printing drum machining processes is discussed by Hennig et al in the paper “Laser processing in printform fabrication” Photonic Applications Systems Technologies Conference (PhAST), PTuA4, Baltimore, May 8, 2007.
None of these focused beam techniques allows the possibility of creating arbitrarily shaped 3D structures with smooth surfaces on the drum surface. This applies particularly to the situation where micro-lenses or other light controlling structures with dimensions less than a few 10s of microns are to be made. Such structures require the use of mask projection methods
The techniques of pulsed laser ablation by mask projection are well known. A mask is illuminated by the beam from a pulsed laser which has passed through optics to reshape it and make it as uniform as possible. A projection lens is used to form a reduced size image of the mask pattern on the surface of a substrate. The lens creates an image of such a size that the energy density of each pulse of laser radiation exceeds the threshold for ablation of the substrate material. The material is ablated at a rate that is usually a fraction of a micron per laser shot and therefore multiple laser shots are needed to create a structure of the required depth. Highly multimode lasers or lasers where the coherence has been reduced are usually used for this mask projection process to avoid interference effects at the image.
It is possible to use these laser ablation mask projection techniques to create 3 dimensional structures in substrates but for these to be of exactly the desired shape it is necessary to change the mask after each laser shot to correspond to the correct contour of the microstructure. Moving a mask is a slow process so ways have been devised to avoid any mask motion yet at the same time create the required 3D structures.
Many examples of this prior art have been disclosed. All use image reduction mask projection systems with static masks that have lines of different features each representing a different contour at different depths in the micro-structure required. The mask features are on a constant pitch and the substrate is moved in the direction parallel to the lines of these features such that every time the laser emits a pulse of radiation the substrate has moved by exactly one or a multiple of the feature pitch distance on the substrate. Because of the requirement to keep the substrate motion exactly in step with the laser firing, this technique can be termed laser pulse synchronized substrate motion (LPSSM).
WO 94/25259 describes a method and apparatus for making arrays of identical through holes of arbitrary shape in polymer films to form thin meshes for medical devices using the LPSSM technique. WO 96/33839 describes a UV excimer laser based LPSSM method and apparatus for making 2 dimensional arrays of identical micro-structures having arbitrary 3D shape on the surface of flat substrates.
Many published articles describe various applications of this LPSSM technique. SPIE Proceedings, Vol 4760, p 281 (2002) names the technique “synchronized image scanning” and shows how it can be used to manufacture ink jet print head plates with extended rows of nozzles with controlled 3D profile. Thin Solid Films, Vol 453-454, p 450 (2004) shows how, as well as linear structures such as ink jet heads, the technique can be used to make arrays of microstructures over large area flat substrates to form masters for subsequent replication. SPIE Proceedings, Vol 5339, p 118 (2004) shows an example of an apparatus that is used to make large area masters for micro-lens arrays on flat plastic substrates by LPSSM. This article raises the issue of problems associated with visible seam artefact defects that occur at the boundary lines between process bands on large area substrates.
WO 2007/135379 A2 introduces new LPSSM methods to overcome visible seam defects on large area flat substrates. This prior art also incorporates half tone edge features into the mask structures in order to eliminate surface discontinuities on the laser ablated microstructures caused by the sharp edges on binary masks.
A key feature of the LPSSM prior art discussed is that in all cases the substrates used have been flat. Precision motion of flat substrates is not a problem for the situation where small devices such as ink jet print heads or medical mesh filters are made by LPSSM since stage travel requirements are small. On the other hand, for the case of the large flat substrates used to make masters for light control and micro-lens array film replication, controlled motion of the substrate becomes a major issue since the manufacture of such devices by LPSSM requires sub micron resolution and micron accuracy over distances in 2 dimensions far exceeding one meter.
For this major application of LPSSM the use of a flat master is not ideal since most industrial high volume film replication production lines use drums to transfer the microstructure to the film. Conversion of a flat master into a cylindrical tool involves many intermediate steps and gives rise to a drum with a linear seam so replicated parts are limited in length to the circumference of the drum. Hence, there remains a need to be able to create precision arrays of 3D microstructures directly on the outside of cylindrical drums.
Visible band interface defects are not an issue when using LPSSM for the manufacture of linear devices such as inkjet print heads or non optical films such as medical meshes. They are however a serious problem when LPSSM methods have been used to make the large masters required for light control and micro-lens array film replication. There is, therefore, a major requirement to devise a method that allows the manufacture of visibly seamless arrays over areas up to several square meters.
Up to now all LPSSM techniques for the manufacture of masters for optical devices have used excimer lasers. These use a mixture of active and buffer gases as the gain medium. Process times for the largest area masters may take many hours over which time the active component in the gas mixture becomes depleted and replacement gas injections are periodically needed. During each injection cycle many of the excimer laser properties such as beam profile, divergence, pulse length, etc change and so the continuous accurate control of the ablation process at the substrate surface is very difficult. Visible discontinuities often arise on the substrate surface when excimer laser operation is interrupted for gas management activities and may also appear during operation due to gas fill degradation. Hence, there is a need, in the case where masters for optical devices are manufactured and where process times are very long, to use alternative lasers that operate with a much higher level of stability.
The present invention addresses the issues discussed above that limit the use of LPSSM for the manufacture of masters for the replication of light control and micro-lens array films.